1. Field of the Invention
The present invention relates to a system for controlling a key storage unit for storing main storage protection keys, reference bits, and change bits corresponding to blocks of a main storage unit.
2. Description of the Prior Art
In a digital processing system, a key storage unit may be used for protecting the computer programs or other contents in a main storage unit. In such a key storge unit, a main storage protection key, a reference bit, and a change bit corresponding to each block of the main storage unit are stored. When a central processing unit desires to access an address in a block of the main storage unit, the main storage protection key stored in the key storage unit is compared with a key presented by the central processing unit. If these two keys coincide, the central processing unit can access the address in the block. If these two keys do not coincide, however, the central processing unit cannot access the address in the block.
In the above-described key storage unit, when the data stored in a block of the main storage unit is read (fetched) by the central processing unit, the reference bit corresponding to that block of the main storage unit must be set to the logic value of "1". When the data originally stored in the block of the main storage unit is re-written (stored) by the central processing unit, both the reference bit and the change bit corresponding to the block of the main storge unit must be set to the logic value of "1". Generally, keys to the main storage unit are provided for every block of 2K bytes or 4K bytes, and every time data in the main storage unit is fetched or stored, the reference bit and the change bit in the key corresponding to the block containing the fetched or stored data must be renewed as described above.
Data access to the main storage unit is usually executed by interleave control. However, the use of interleave control for data access to the key storage unit makes the control process complicated, increases the amount of the hardware, and increases the cost. Thus, the key storage unit must be constructed of a memory device with a faster access time than the main storage unit. For example, if the main storage unit is constructed by a metal oxide semiconductor (MOS) random access memory (RAM) device, the key storage unit is constructed of a dynamic RAM device. However, the greater the capacity of the main storage unit, the more difficult the construction of the key storage unit and the higher the cost.